Honeycomb structure

ABSTRACT

A honeycomb structure having multiple honeycomb units united through a seal material layer, wherein each honeycomb unit has multiple through holes arranged side by side in a longitudinal direction and separated from each other by the wall surfaces of the through holes, is disclosed. The honeycomb units include at least: ceramic particles; and at least one of inorganic fibers and whiskers. At least one of the honeycomb units has a cross section perpendicular to a longitudinal direction thereof, the cross section having an area greater than or equal to about 5 cm 2  and less than or equal to about 50 cm 2 . Each corner of each honeycomb unit has a shape of one of a substantially rounded surface and a substantially chamfered surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a honeycomb structure.

2. Description of the Related Art

Conventionally, a common honeycomb catalyst used for conversion ofautomotive exhaust gas is manufactured by having a high specific surfacearea material such as activated alumina and catalyst metal such asplatinum carried on the surface of a monolithic cordierite-basedhoneycomb structure having a low thermal expansion characteristic.Further, alkaline earth metal such as Ba is carried as a NO_(x)occlusion agent for processing NO, in an oxygen-excess atmosphere suchas a lean-burn engine or a diesel engine. In order to achieve a furtherimprovement in conversion performance, it is necessary to increase theprobability of contact of exhaust gas and the catalyst noble metal andthe NO_(x) occlusion agent. This requires the carrier to have a higherspecific surface area and the particles of the noble metal to be reducedin size and highly dispersed. However, a mere increase in the carriedamount of a high specific surface area material such as activatedalumina only results in an increase in the thickness of the aluminalayer, thus causing a problem in that the probability of contact is notincreased or that pressure loss is too high. Accordingly, somemodifications have been made on cell shape, cell density, and wallthickness (for example, see JP-A 10-263416). On the other hand, as ahoneycomb structure formed of a high specific surface area material, ahoneycomb structure formed by extrusion molding using a compositionincluding inorganic fibers and an inorganic binder is known (forexample, see JP-A 5-213681). Further, a honeycomb structure formed byjoining honeycomb units through an adhesion layer in order to increasethe size of such a honeycomb structure is known (for example, see DE4341159 A1).

The contents of JP-A 10-263416, JP-A 5-213681, and DE 4341159 A1 areincorporated herein by reference in their entirety.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, provided is ahoneycomb structure having a plurality of honeycomb units united througha seal material layer, the honeycomb units each having a plurality ofthrough holes arranged side by side in a longitudinal direction andseparated from each other by wall surfaces of the through holes, whereinthe honeycomb units include at least ceramic particles and at least oneof inorganic fibers and whiskers, at least one of the honeycomb unitshas a cross section perpendicular to a longitudinal direction thereof,the cross section having an area greater than or equal to about 5 cm²and less than or equal to about 50 cm²; and each of corners of eachhoneycomb unit has a shape of one of a substantially rounded surface anda substantially chamfered surface.

Thus, according to one embodiment of the present invention, a honeycombstructure capable of achieving high dispersion of a catalyst componentand increasing strength against thermal shock and vibration may beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1A is a conceptual diagram showing a honeycomb structure accordingto an embodiment of the present invention;

FIG. 1B is a diagram showing a joint body of honeycomb units accordingto the embodiment of the present invention;

FIG. 2A is a conceptual diagram showing the honeycomb unit according tothe embodiment of the present invention;

FIGS. 2B through 2E are diagrams for illustrating the shapes of thecorners of the honeycomb units according to the embodiment of thepresent invention;

FIG. 3 is an SEM photograph of the wall surface of the honeycomb unitaccording to the embodiment of the present invention;

FIG. 4A is a diagram for illustrating experimental examples in which thehoneycomb units were joined according to the embodiment of the presentinvention;

FIG. 4B is a diagram for illustrating other experimental examples inwhich the honeycomb units were joined according to the embodiment of thepresent invention;

FIG. 4C is a diagram for illustrating other experimental examples inwhich the honeycomb units were joined according to the embodiment of thepresent invention;

FIG. 4D is a diagram for illustrating other experimental examples inwhich the honeycomb units were joined according to the embodiment of thepresent invention;

FIG. 5A is a diagram for illustrating other experimental examples inwhich the honeycomb units were joined according to the embodiment of thepresent invention;

FIG. 5B is a diagram for illustrating other experimental examples inwhich the honeycomb units were joined according to the embodiment of thepresent invention;

FIG. 5C is a diagram for illustrating other experimental examples of thehoneycomb unit according to the embodiment of the present invention;

FIG. 6A is a front view of a vibration apparatus according to theembodiment of the present invention;

FIG. 6B is a side view of the vibration apparatus according to theembodiment of the present invention;

FIG. 7 is a diagram for illustrating a pressure loss measuring apparatusaccording to the embodiment of the present invention;

FIG. 8 is a graph showing the relationship between the cross-sectionalarea of the honeycomb unit and each of weight reduction rate andpressure loss according to the embodiment of the present invention;

FIG. 9 is a graph showing the relationship between unit area ratio andeach of weight reduction rate and pressure loss according to theembodiment of the present invention; and

FIG. 10 is a graph showing the relationship between the aspect ratio ofsilica-alumina fibers and weight reduction rate according to theembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, a description is given, with reference to the accompanyingdrawings, of an embodiment of the present invention.

FIG. 1A is a conceptual diagram of a honeycomb structure 10 according tothe embodiment of the present invention. Referring to FIG. 1A, thehoneycomb structure 10 according to this embodiment is formed by unitingmultiple honeycomb units 11 through a seal material layer 14. Eachhoneycomb unit 11 has multiple through holes 12 arranged side by side ina longitudinal direction, being separated by through hole wall surfaces.Each honeycomb unit 11 includes at least ceramic particles and inorganicfibers and/or whiskers. The area of a cross section of the honeycombunit 11 perpendicular to a longitudinal direction thereof is greaterthan or equal to about 5 cm² and less than or equal to about 50 m².Corners 18 (FIGS. 2A through 2C) of the honeycomb unit 11 havesubstantially rounded-surface and/or substantially chamfered-surfaceshapes.

This honeycomb structure 10 has the multiple honeycomb units 11 joinedthrough the seal material layer 14. Accordingly, it is possible toincrease strength against thermal shock and vibration. It is inferredthat this is because it is possible to reduce a difference intemperature in each honeycomb unit 11 even when there is a distributionof temperature in the honeycomb structure 10 because of a sudden changein temperature, or because it is possible to ease thermal shock andvibration with the seal material layer 14. Further, it is believed thatalso in the case of formation of cracks in one or more of the honeycombunits 11 due to thermal stress, the seal material layer 14 prevents thecracks from extending to the entire honeycomb structure 10 and serves asthe frame of the honeycomb structure 10 to maintain the shape as ahoneycomb structure, thereby preventing the honeycomb structure 10 fromlosing the ability to function as a catalyst carrier. With respect tothe size of each honeycomb unit 11, if the area of a cross section ofthe honeycomb unit 11 perpendicular to its through holes 12(hereinafter, simply referred to as “cross-sectional area”) is greaterthan or equal to about 5 cm², the cross-sectional area of the sealmaterial layer 14 joining the multiple honeycomb units 11 decreases soas to be able to relatively increase the specific surface area carryinga catalyst and relatively reduce pressure loss. On the other hand, ifthe cross-sectional area of the honeycomb unit 11 is less than or equalto about 50 cm², the honeycomb unit 11 is not too large in size, andaccordingly, can sufficiently control thermal stress generated therein.That is, with a cross-sectional area of about 5 cm² to about 50 cm²,each honeycomb unit 11 is at a practicable level, having reducedpressure loss, sufficient strength against thermal stress, and highdurability while maintaining a large specific surface area. Therefore,according to this honeycomb structure 10, it is possible to cause acatalyst component to be highly dispersed, and to increase strengthagainst thermal shock and vibration. If the honeycomb structure 10includes multiple types of honeycomb units having differentcross-sectional areas, the term “cross-sectional area” refers to thecross-sectional area of a honeycomb unit that is a basic unit of thehoneycomb structure 10, and usually refers to the largest one of thecross-sectional areas of the honeycomb units.

Further, it is preferable that the radius of curvature R (indicated by ain FIG. 2B) of the rounded-surfaces of the corners 18 of each honeycombunit 11 be about 0.3 mm to about 2.5 mm (0.3 mm≦a≦2.5 mm).

In the honeycomb structure 10 of this embodiment, since the corners 18of each honeycomb unit 11 on its exterior surface have substantiallyrounded-surface and/or substantially chamfered-surface shapes, it ispossible to avoid concentration of stress on the corners 18.Accordingly, it is possible to prevent chipping from being caused in thehoneycomb unit 11 and to prevent cracks from being caused and spreadingin the seal material layer 14 from the corners 18 as starting points.Preferably, the radius of curvature R of the rounded-surface shapes isabout 0.3 mm to about 2.5 mm. If the radius of curvature R is greaterthan or equal to about 0.3 mm, concentration of stress on the corners 18can be avoided sufficiently, so that chipping and cracks are less likelyto be caused. If the radius of curvature R is less than or equal toabout 2.5 mm, the cross-sectional area of the honeycomb unit 11increases, so that the processing capability of the honeycomb structure10 is less likely to be reduced.

Further, it is preferable that the chamfered-surface shapes be about 0.3mm to about 2.5 mm in size (indicated by b in FIG. 2C, 0.3 mm≦b≦2.5 mm)because this eases concentration of stress on the edge parts of thehoneycomb structure 10, thus increasing its strength. If thechamfered-surface shapes are greater than or equal to about 0.3 mm,concentration of stress on the corners 18 can be avoided sufficiently,so that chipping and cracks are less likely to be caused. Further, ifthe chamfered-surface shapes are less than or equal to about 2.5 mm, thedifference in thickness of the seal material does not increase, so thatthe honeycomb structure 10 is less likely to be broken because ofthermal stress. Further, the cross-sectional area of the honeycomb unit11 increases, so that the processing capability of the honeycombstructure 10 is less likely to be reduced.

As described above, the corners 18 of the honeycomb unit 11 havesubstantially rounded-surface and/or substantially chamfered-surfaceshapes. Here, by the term “substantially,” it is intended that thesurface shape of each corner 18 may not be precisely rounded orchamfered, and that the rounded-surface or chamfered-surface shape ofeach corner 18 may include a shape similar thereto as long as the shapeproduces the same effect as described above (avoiding concentration ofstress on the corners 18). For example, as shown in FIG. 2D, therounded-surface shape of the corner 18 may include unevenness. Further,as shown in FIG. 2E, the chamfered-surface shape of the corner 18 mayfurther include a rounded surface 18 a and a chamfered surface 18 b aslong as this shape produces the same effect as described above. In thiscase, the rounded surface 18 a may be replaced by a chamfered surface,and the chamfered surface 18 b may be replaced by a rounded surface.Further, in FIG. 2C, the triangle formed by the chamfered surface of thecorner 18 (hypotenuse) and the two sides indicated by b (broken lines)may be any right triangle as long as the length of each side indicatedby b falls within the above-described range of about 0.3 mm to about 2.5mm.

The ratio of the total of the cross-sectional areas of the honeycombunits 11 (the areas of cross sections of the honeycomb units 11perpendicular to a longitudinal direction thereof) to thecross-sectional area of the honeycomb structure 10 (the area of a crosssection of the honeycomb structure 10 perpendicular to a longitudinaldirection thereof) is preferably greater than or equal to about 85%, andmore preferably, greater than or equal to about 90%. If this ratio isgreater than or equal to about 85%, the cross-sectional area of the sealmaterial layer 14 decreases so as to increase the total cross-sectionalarea of the honeycomb units 11. Therefore, it is possible to relativelyincrease the specific surface area carrying a catalyst, and torelatively reduce pressure loss. When this ratio is greater than orequal to about 90%, it is possible to further reduce pressure loss.

The honeycomb structure 10 according to this embodiment may include acoating material layer 16 (FIG. 1A) coating an exterior axial surface 15(exterior cylindrical surface in the case of FIG. 1B) of the joint bodyof the honeycomb units 11, or the exterior surface of the joint body ofthe honeycomb units 11 which surface is along the axial directions ofthe joint body. This protects the exterior cylindrical surface of thehoneycomb structure 10, so that its strength can be increased.

The honeycomb structure 10 into which the honeycomb units 11 are joinedis not limited in shape in particular, and may also be shaped like, forexample, a prism.

In the honeycomb structure 10 of this embodiment, the aspect ratio ofinorganic fibers and/or whiskers included in each honeycomb unit 11 ispreferably about 2 to about 1000, more preferably, about 5 to about 800,and most preferably, about 10 to about 500. If the aspect ratio ofinorganic fibers and/or whiskers is greater than or equal to about 2, itis possible to increase their contribution to an increase in thestrength of the honeycomb structure. On the other hand, aspect ratiosless than or equal to about 1000 are less likely to cause clogging of adie for extrusion molding at the time of molding, thus resulting in goodmoldability. Further, at the time of molding such as extrusion molding,the inorganic fibers and/or whiskers are less likely to break, andaccordingly, are less likely to vary in length, thus increasing theircontribution to an increase in the strength of the honeycomb structure10. If the inorganic fibers and/or whiskers have their aspect ratiosdistributed, the aspect ratio of inorganic fibers and/or whiskers may betheir average.

In the honeycomb structure 10 of this embodiment, the ceramic particlesincluded in each honeycomb unit 11 are not limited in particular. Theceramic particles include one or more selected from, for example,silicon carbide, silicon nitride, alumina, silica, zirconia, titania,ceria, mullite, and zeolite. Of these, alumina is preferred.

In the honeycomb structure 10 of this embodiment, the inorganic fibersand/or whiskers included in each honeycomb unit 11 are not limited inparticular. The inorganic fibers and/or whiskers include one or moreselected from alumina, silica, silicon carbide, silica alumina, aluminumborate, glass, and potassium titanate.

The ceramic particles included in the honeycomb structure 10 arepreferably about 30 wt % to about 97 wt %, more preferably about 30 wt %to about 90 wt %, still more preferably about 40 wt % to about 80 wt %,and most preferably about 50 wt % to about 75 wt % in amount. If theceramic particles content is greater than or equal to about 30 wt %, theceramic particles contributing to an increase in the specific surfacearea relatively increase in amount. This increases the specific surfacearea as a honeycomb structure so as to make it possible to carry acatalyst component with high dispersion easily. If the ceramic particlescontent is less than or equal to about 90 wt %, the inorganic fibersand/or whiskers contributing to an increase in strength relativelyincrease in amount, so that the strength of the honeycomb structure 10is less likely to be reduced.

The inorganic fibers and/or whiskers included in the honeycomb units 11of the honeycomb structure 10 are preferably about 3 wt % to about 70 wt%, more preferably about 3 wt % to about 50 wt %, still more preferablyabout 5 wt % to about 40%, and most preferably about 8 wt % to about 30wt % in amount. If the inorganic fibers and/or whiskers content isgreater than or equal to about 3 wt %, the strength of the honeycombstructure 10 is less likely to be reduced. If the inorganic fibersand/or whiskers content is less than or equal to about 50 wt %, theceramic particles contributing to an increase in the specific surfacearea relatively increase in amount. This increases the specific surfacearea as a honeycomb structure so as to make it possible to carry acatalyst component with high dispersion easily.

In the honeycomb structure 10 of this embodiment, each honeycomb unit 11may also be manufactured with further inclusion of an inorganic binder.This makes it possible to obtain sufficient strength even when firing isperformed on the honeycomb unit 11 at lower temperatures. The inorganicbinder included in the honeycomb structure 10 is not limited inparticular. The inorganic binder may be, for example, inorganic sol, aclay-based binder, etc. Of these, the inorganic sol includes one or moreselected from, for example, alumina sol, silica sol, titania sol, andwater glass. The clay-based binder includes one or more clay-basedbinders selected from, for example, clay, kaolin, montmonrillonite, andclays of a double-chain structure type (sepiolite and attapulgite). Theinorganic binder included at the time of manufacturing the honeycombunits 11 is preferably less than or equal to about 50 wt %, morepreferably about 5 wt % to about 50 wt %, still more preferably about 10wt % to about 40 wt %, and most preferably about 15 wt % to about 35 wt% in amount as a solid content included in the honeycomb structure 10.If the inorganic binder content is less than or equal to about 50 wt %in amount, moldability is prevented from being degraded.

The shape of each honeycomb unit 11 is not limited in particular.Preferably, the honeycomb units 11 are shaped so as to facilitatejoining of the honeycomb units 11. A section of the honeycomb unit 11perpendicular to the through holes 12 (hereinafter, simply referred toas “cross section”) may be square or rectangular. FIG. 2 is a conceptualdiagram of an example of the honeycomb units 11, which is aparallelepiped one having a square cross section. Referring to FIG. 2,the honeycomb unit 11 includes the multiple through holes 12 extendingfrom the front side to the rear side, and an exterior surface 13 havingno openings of the through holes 12. The thickness of the wall betweenthe through holes 12 is not limited in particular. The wall thickness ispreferably about 0.05 mm to about 0.35 mm, more preferably about 0.10 mmto about 0.30 mm, and most preferably about 0.15 mm to about 0.25 mm. Ifthe wall thickness is greater than or equal to about 0.05 mm, thestrength of the honeycomb unit 11 is less likely to be reduced. On theother hand, if the wall thickness is less than or equal to about 0.35mm, the area of contact with exhaust gas increases, so that the exhaustgas is likely to penetrate sufficiently deeply. As a result, a catalystcarried on the inside of the wall is likely to come into contact withthe exhaust gas, so that it is possible to improve catalyst performance.Further, the number of through holes 12 per unit cross-sectional area ispreferably about 15.5/cm² to about 186/cm² (about 100 cpsi to about 1200cpsi), more preferably about 46.5/cm² to about 170.5/cm² (about 300 cpsito about 1100 cpsi), and most preferably about 62.0/cm² to about 155/cm²(about 400 cpsi to about 1000 cpsi). If the number of through holes 12is greater than or equal to about 15.5/cm², the area of the wall insidethe honeycomb unit 11 which wall contacts exhaust gas increases. If thenumber of through holes 12 is less than or equal to about 186/cm²,pressure loss is less likely to increase, and it is easy to manufacturethe honeycomb unit 11.

Each honeycomb unit 11 forming the honeycomb structure 10 may be of suchsize as to have a cross-sectional area of preferably about 5 cm² toabout 50 cm, more preferably about 6 cm² to about 40 cm², and mostpreferably about 8 cm² to about 30 cm². If the cross-sectional areafalls within the range of about 5 cm² to about 50 cm², it is possible tocontrol the ratio of the seal material layer 14 to the honeycombstructure 10. This makes it possible to keep high the specific surfacearea per unit volume of the honeycomb structure 10, and accordingly, tohave a catalyst component highly dispersed. This also makes it possibleto maintain the shape as a honeycomb structure even if an external forcesuch as thermal shock or vibration is applied. The specific surface areaper unit volume can be given by Eq. (1) described below.

An object of the present invention may be to provide a honeycombstructure capable of achieving high dispersion of a catalyst componentand increasing strength against thermal shock and vibration.

According to one embodiment of the present invention, a carrier can havea higher specific surface area, and the particles of a catalyst metalcan be reduced in size and can be more highly dispersed. That is, it ispossible to increase the specific surface area of a high specificsurface area material such as alumina at its initial stage. Accordingly,even if sintering of a high specific surface area material progressesbecause of thermal aging (use as a catalyst carrier), and with this, acatalyst metal such as platinum carried thereon coheres so as toincrease in particle size, it is possible to maintain a high specificsurface area. As a result, it is possible to increase the probability ofcontact of exhaust gas with a catalyst noble metal and a NO_(x)occlusion agent and thus to achieve a further improvement in conversionperformance without carrying a large amount of noble metal such asplatinum, which is very expensive and a limited valuable resource, ascatalyst metal or increasing the size of a catalyst carrier itself.Further, since a catalyst carrier itself is not increased in size, it issuitable for installation in automobiles, which are limited in space forinstallation. Further, since it is possible to increase the specificsurface area of the carrier without reducing the thickness of itspartition walls, sufficient strength can be obtained. Further, ahoneycomb structure has multiple honeycomb units joined through a sealmaterial layer. This makes it possible to reduce a difference intemperature in each honeycomb unit even when there is a distribution oftemperature in the honeycomb structure because of a sudden change intemperature. This also makes it possible to ease thermal shock andvibration with the seal material layer. Further, in the case offormation of cracks in one or more of the honeycomb units due to thermalstress, the seal material layer prevents the cracks from extending tothe entire honeycomb structure and serves as the frame of the honeycombstructure to maintain the shape as a honeycomb structure. Accordingly,even if thermal stress due to a sudden change in temperature or externalforce such as great vibration is applied in the case of, for example,use for automobiles, the honeycomb structure is prevented from breakingeasily, and can maintain the shape as a honeycomb structure andaccordingly, function as a catalyst carrier. Further, in the honeycombstructure, since the corners of each honeycomb unit on its exteriorsurface have substantially rounded-surface and/or substantiallychamfered-surface shapes, it is possible to avoid concentration ofstress on the corners. Accordingly, it is possible to prevent chippingfrom being caused in the honeycomb unit and to prevent cracks from beingcaused and spreading in the seal material layer from the corners asstarting points. Therefore, breakage of the honeycomb structure, as wellas a decrease in processing efficiency due to exhaust gas leakage, canbe prevented.

Next, a description is given of a method of manufacturing-theabove-described honeycomb structure 10 of this embodiment. First,honeycomb unit molded bodies are made by extrusion molding using rawmaterial paste including the above-described ceramic particles,inorganic fibers and/or whiskers, and inorganic binder as principalcomponents. In addition to these; an organic binder, a dispersionmedium, and a molding aid may be added appropriately to the raw materialpaste in accordance with moldability. The organic binder is not limitedin particular. The organic binder includes one or more selected from,for instance, methylcellulose, carboxymethylcellulose,hydroxyethylcellulose, polyethylene glycol, phenolic resin, and epoxyresin. The mix proportion of the organic binder is preferably about 1part to about 10 parts by weight to the total of 100 parts by weight ofthe ceramic particles, the inorganic fibers and/or whiskers, and theinorganic binder. The dispersion medium is not limited in particular,and may be, for example, water, an organic solvent (such as benzene),alcohol (such as methanol), etc. The molding aid is not limited inparticular, and may be, for example, ethylene glycol, dextrin, a fattyacid, fatty acid soap, and polyalcohol.

The raw material paste is not limited in particular. It is preferable tomix and knead the raw material paste. For example, the raw materialpaste may be mixed using a mixer or attritor, and may be well kneadedwith a kneader. The method of molding the raw material paste is notlimited in particular. Preferably, the raw material paste is formed intoa shape having through holes by, for example, extrusion molding or thelike.

The corners 18 of the honeycomb units 11 may be provided withsubstantially rounded-surface and/or substantially chamfered-surfaceshapes by providing the exterior surface of a mold used for theextrusion molding with a substantially rounded-surface and/orsubstantially chamfered-surface shape.

The corners 18 of the honeycomb units 11 may be formed intorounded-surface shapes of a radius of curvature R and/orchamfered-surface shapes of a predetermined size by chamfering such asgrinding or cutting. FIG. 2B is a front view of the honeycomb unit 11 inwhich the corners 18 are provided with rounded-surface shapes. FIG. 2Cis a front view of the honeycomb unit 11 in which the corners 18 areprovided with chamfered-surface shapes. The stage of chamfering is notlimited to this time point, and chamfering may be performed, forexample, after firing. In this case, it is desirable to provide the unitcorners with a large film thickness in advance so as to prevent the unitcorners 18 from becoming thin.

Next, it is preferable to dry the obtained molded bodies. The drier usedfor drying is not limited in particular, and may be a microwave drier,hot air drier, dielectric drier, reduced-pressure drier, vacuum drier,and freeze drier. Further, it is preferable to degrease the obtainedmolded bodies. The conditions for degreasing are not limited inparticular, and are selected suitably in accordance with the organicmatter included in the molded bodies. Preferably, the conditions are aheating temperature of approximately 400° C. and a heating time ofapproximately 2 hours.

Further, it is preferable to subject the obtained molded bodies tofiring. The condition for firing is not limited in particular, and ispreferably at about 600° C. to about 1200° C., and more preferably atabout 600° C. to about 1000° C. This is because sintering of the ceramicparticles progresses sufficiently at firing temperatures higher than orequal to about 600° C., thus preventing strength as a honeycombstructure from being reduced, and because sintering of the ceramicparticles does not progress excessively at firing temperatures lowerthan or equal to about 1200° C., thus increasing the specific surfacearea per unit volume, so that a carried catalyst component can bedispersed highly enough with ease. By way of these processes, thehoneycomb units 11 each having the multiple through holes 12 can beobtained.

Next, the paste of a seal material to serve as the seal material layer14 is applied on the obtained honeycomb units 11, and the honeycombunits 11 are successively joined. Thereafter, the honeycomb units 11 maybe dried and solidified so as to be formed into a honeycomb unit jointbody of a predetermined size. The seal material is not limited inparticular. For example, a mixture of an inorganic binder and ceramicparticles, a mixture of an inorganic binder and inorganic fibers, and amixture of an inorganic binder, ceramic particles, and inorganic fibersmay be used as the seal material. An organic binder may be added tothese seal materials. The organic binder is not limited in particular,and includes one or more selected from, for example, polyvinyl alcohol,methylcellulose, ethylcellulose, and carboxymethylcellulose.

The seal material layer 14 joining the honeycomb units 11 is preferablyabout 0.5 mm to about 2 mm in thickness. If the seal material layer 14is greater than or equal to about 0.5 mm in thickness, sufficientjoining strength is likely to be obtained. Further, the seal materiallayer 14 is a part that does not function as a catalyst carrier.Accordingly, if the seal material layer 14 is less than or equal toabout 2 mm in thickness, the specific surface area per unit volume ofthe honeycomb structure 10 is less likely to be reduced, so that acatalyst component can be carried with sufficiently high dispersion withease. Further, if the seal material layer 14 is less than or equal toabout 2 mm in thickness, pressure loss is less likely to be increased.The number of honeycomb units 11 to be joined may be determined suitablyin accordance with the size of the honeycomb structure 10 to be used asa honeycomb catalyst. Further, the joint body into which the honeycombunits 11 are joined with the seal material may also be cut and groundsuitably in accordance with the shape and size of the honeycombstructure 10, for example, as shown in FIG. 1B. FIG. 1B is a perspectiveview of the joint body of the honeycomb units 11 after cutting andgrinding.

The coating material layer 16 may be formed by applying a coatingmaterial on the exterior cylindrical surface (side surface) 15 of thejoint body of the honeycomb units 11, and drying and solidifying thecoating material. This makes it possible to increase strength byprotecting the exterior cylindrical surface of the honeycomb structure10.

The coating material is not limited in particular, and may be eitherequal to or different from the seal material in material. Further, thecoating material may be equal to or different from the seal material incompounding ratio. The thickness of the coating material layer 16 is notlimited in particular, and is preferably about 0.1 mm to about 2 mm.With thicknesses greater than or equal to about 0.1 mm, the coatingmaterial layer 16 can sufficiently protect the exterior cylindricalsurface of the honeycomb structure 10, so that it is possible toincrease its strength with ease. If the coating material layer 16 isless than or equal to about 2 mm in thickness, the specific surface areaper unit volume as a honeycomb structure is less likely to be reduced sothat it is possible to carry a catalyst component with sufficiently highdispersion easily.

It is preferable to perform calcination after joining the multiplehoneycomb units 11 with the seal material (or after the coating materiallayer 16 is formed in the case of providing the coating material layer16). This is because if an organic binder is included in the sealmaterial or the coating material, the organic material can be removed bydegreasing. The conditions for calcination may be determined suitably inaccordance with the type and amount of the included organic binder.Preferably, the calcination is performed for approximately 2 hours atapproximately 700° C. When the honeycomb structure 10 obtained by thecalcination is used, there is no emission of contaminated exhaust gasdue to a remaining organic binder in the honeycomb structure 10.Referring back to FIG. 1A, as an example honeycomb structure, thehoneycomb structure 10 of this embodiment is formed so as to have acylindrical shape by joining the multiple parallelepiped honeycomb units11 each having a square cross section. The honeycomb structure 10 isformed by joining the honeycomb units 11 with the seal material layer14, cutting the joined honeycomb units 11 into a cylindrical shape asshown in FIG. 1B, and thereafter, coating the exterior cylindricalsurface 15 of the joint body of the honeycomb units 11 with the coatingmaterial layer 16. The unevenness (not graphically illustrated in FIG.1B) of the exterior cylindrical surface 15 of the joint body of thehoneycomb units 11 due to the cutting or grinding is eliminated byfilling the concave parts of the exterior cylindrical surface 15 withthe coating material layer 16. Optionally, the concave parts of theexterior cylindrical surface 15 may not be filled with the coatingmaterial layer 16. Alternatively, the honeycomb units 11 may be moldedso as to have fan-shaped and square cross sections, and joined into apredetermined honeycomb structure shape (a cylindrical shape in FIG.1A), thereby omitting the cutting and grinding process.

The obtained honeycomb structure 10 is not limited to a particular use.It is preferable to use the obtained honeycomb structure 10 as acatalyst carrier for converting the exhaust gas of vehicles. In the caseof using the honeycomb structure 10 as a catalyst carrier for convertingthe exhaust gas of a diesel engine, the honeycomb structure 10 may beused with a diesel particulate filter (DPF) having a ceramic honeycombstructure of silicon carbide or the like and having the function ofpurifying the exhaust gas by filtering out and burning particulatematter (PM) therein. In this case, the positional relationship betweenthe honeycomb structure 10 of this embodiment and the DPF is such thatthe honeycomb structure 10 of this embodiment may be provided eitherbefore or after the DPF. In the case of providing the honeycombstructure 10 before the DPF, when the honeycomb structure 10 shows areaction accompanied by heat generation, the generated heat is conductedto the subsequent DPF, so that it is possible to promote temperaturerising at the time of the regeneration of the DPF. In the case ofproviding the honeycomb structure 10 after the DPF, the exhaust gaspasses through the through holes 12 of the honeycomb structure 10 afterthe PM in the exhaust gas is filtered out by the DPF. Accordingly,clogging is less likely to occur in the honeycomb structure 10. Further,a gas component generated by incomplete combustion at the time ofburning the PM in the DPF can also be processed using the honeycombstructure 10 of this embodiment. The honeycomb structure 10 may be usedfor the purpose described above with reference to the background of thepresent invention. In addition, the honeycomb structure 10 may also beapplied to, but is not limited in particular to, use without carrying acatalyst component (for example, as an adsorbent adsorbing a gascomponent or a liquid component).

Further, the obtained honeycomb structure 10 may be made a honeycombcatalyst by having a catalyst component carried thereon. The catalystcomponent is not limited in particular, and may be noble metal, alkalimetal, alkaline earth metal, oxide, etc. The noble metal includes one ormore selected from, for example, platinum, palladium, and rhodium. Thealkali metal includes one or more selected from, for example, potassiumand sodium. Examples of the alkaline earth metal include barium.Examples of the oxide include perovskite (such as La_(0.75)K_(0.25)MnO₃)and CeO₂. The obtained honeycomb catalyst is not limited to a particularuse, and may be used as, for example, a so-called three-way catalyst ora NO_(x) occlusion catalyst for converting automobile exhaust gas. Thereis no particular limitation with respect to the carrying of a catalystcomponent. The catalyst component may be carried either aftermanufacturing the honeycomb structure 10 or at the stage of raw materialceramic particles. The method of carrying a catalyst component is notlimited in particular, and may be, for example, impregnation.

A description is given below of the specific cases of manufacturinghoneycomb structures under various conditions as experimental examples.The present invention is not limited to these experimental examples.

EXAMPLE 1

First, 40 wt % of γ-alumina particles (2 μm in average particle size),10 wt % of silica-alumina fibers (10 μm in average fiber diameter, 100μm in average fiber length, and of an aspect ratio of 10), and 50 wt %of silica sol (of a solid concentration of 30 wt %) were mixed. Sixparts by weight of methylcellulose as an organic binder was added,together with small amounts of a plasticizer and lubricant, to 100 partsby weight of the obtained mixture. The mixture was further mixed andkneaded, so that a mixture composition was obtained. Next, the mixturecomposition was subjected to extrusion molding by an extruder, so thatcrude molded bodies were obtained.

The extrusion molding was performed using such a mold as to form arounded-surface shape of a radius of curvature R of 1.5 mm on eachcorner 18 of the honeycomb units 11.

The crude molded bodies were sufficiently dried using a microwave drierand a hot air drier, and were degreased, being heated at 400° C. for 2hours.

Thereafter, the molded bodies were subjected to firing, being heated at800° C. for 2 hours. As a result, the honeycomb units 11 each having aprism-like shape (34.3 mm×34.3 mm×150 mm), a cell density of 93/cm² (600cpsi), a wall thickness of 0.2 mm, and a quadrangular (square) cellshape were obtained. FIG. 3 shows a scanning electron microscope (SEM)photograph of the wall surface of one of the honeycomb units 11. Thisshows that silica-alumina fibers are oriented along the extrusiondirection of the raw material paste in this honeycomb unit 11.

Next, 29 wt % of γ-alumina particles (2 μm in average particle size), 7wt % of silica-alumina fibers (10 μm in average fiber diameter and 100μm in average fiber length), 34 wt % of silica sol (of a solidconcentration of 30 wt %), 5 wt % of carboxymethylcellulose, and 25 wt %of water were mixed into heat-resisting seal material paste. Thehoneycomb units 11 were joined using this seal material paste. FIG. 4Ashows a joint body into which the honeycomb units 11 are joined, viewedfrom a surface thereof on which the through holes 12 are formed(hereinafter referred to as “front surface”). This joint body was formedby joining and solidifying the honeycomb units 11 with the seal materialpaste being applied on the exterior surfaces 13 of the honeycomb units11 so that the seal material layer 14 was 1 mm in thickness. The jointbody was thus made, and the joint body was cut cylindrically using adiamond cutter so that the front surface of the joint body wassubstantially symmetric about a point. Further, the above-described sealmaterial paste was applied on the exterior cylindrical surface on whichno through holes were formed so as to be 0.5 mm in thickness, therebycoating the exterior cylindrical surface. Thereafter, the obtainedstructure was dried at 120° C., and the seal material layer 14 and thecoating material layer 16 were degreased while heating the structure at700° C. for 2 hours. As a result, the honeycomb structure 10 having acylindrical shape (143.8 mm in diameter and 150 mm in length) wasobtained.

Table 1 shows a collection of data such as numerical values on theceramic particle component, unit shape, unit cross-sectional area, unitarea ratio (the ratio of the total cross-sectional area of the honeycombunits 11 to the cross-sectional area of the honeycomb structure 10,which applies hereinafter), and seal material layer area ratio (theratio of the total cross-sectional area of the seal material layer 14and the coating material layer 16 to the cross-sectional area of thehoneycomb structure 10, which applies hereinafter) of this honeycombstructure 10. TABLE 1 UNIT SEAL²⁾ CROSS- UNIT MATERIAL UNIT SECTIONALAREA LAYER CERAMIC SHAPE AREA RATIO AREA RATIO SAMPLE¹⁾ PARTICLES cm cm²% % EXAMPLE 1 ALUMINA 3.43 SQUARE 11.8 93.5 6.5 EXAMPLE 2 ALUMINA 2.00SQUARE 4.0 89.7 10.3 EXAMPLE 3 ALUMINA 2.24 SQUARE 5.0 90.2 9.8 EXAMPLE4 ALUMINA 7.09 FAN 39.5 96.9 3.1 EXAMPLE 5 ALUMINA 7.10 SQUARE 50.0 95.54.5 EXAMPLE 6 ALUMINA 7.41 SQUARE 55.0 95.6 4.4 EXAMPLE 7 ALUMINAMONOLITHIC 162.0 100.0 0 EXAMPLE 8 TITANIA 3.43 SQUARE 11.8 93.5 6.5EXAMPLE 9 TITANIA 2.00 SQUARE 4.0 89.7 10.3 EXAMPLE 10 TITANIA 2.24SQUARE 5.0 90.2 9.8 EXAMPLE 11 TITANIA 7.09 FAN 39.5 96.9 3.1 EXAMPLE 12TITANIA 7.10 SQUARE 50.0 95.5 4.5 EXAMPLE 13 TITANIA 7.41 SQUARE 55.095.6 4.4 EXAMPLE 14 TITANIA MONOLITHIC 162.0 100.0 0 EXAMPLE 15 SILICA3.43 SQUARE 11.8 93.5 6.5 EXAMPLE 16 SILICA 2.00 SQUARE 4.0 89.7 10.3EXAMPLE 17 SILICA 2.24 SQUARE 5.0 90.2 9.8 EXAMPLE 18 SILICA 7.09 FAN39.5 96.9 3.1 EXAMPLE 19 SILICA 7.10 SQUARE 50.0 95.5 4.5 EXAMPLE 20SILICA 7.41 SQUARE 55.0 95.6 4.4 EXAMPLE 21 SILICA MONOLITHIC 162.0100.0 0 EXAMPLE 22 ZIRCONIA 3.43 SQUARE 11.8 93.5 6.5 EXAMPLE 23ZIRCONIA 2.00 SQUARE 4.0 89.7 10.3 EXAMPLE 24 ZIRCONIA 2.24 SQUARE 5.090.2 9.8 EXAMPLE 25 ZIRCONIA 7.09 FAN 39.5 96.9 3.1 EXAMPLE 26 ZIRCONIA7.10 SQUARE 50.0 95.5 4.5 EXAMPLE 27 ZIRCONIA 7.41 SQUARE 55.0 95.6 4.4EXAMPLE 28 ZIRCONIA MONOLITHIC 162.0 100.0 0 EXAMPLE 29 CORDIERITE +MONOLITHIC 162.0 100.0 0 ALUMINA¹⁾INORGANIC FIBERS = SILICA-ALUMINA FIBERS (10 μm IN DIAMETER, 100 μm INLENGTH, ASPECT RATIO OF 10)²⁾INCLUDING AREA OF COATING MATERIAL LAYER

The contents of Examples 2 through 29 to be described below are alsoshown together in Table 1. In each sample shown in Table 1, theinorganic fibers-are silica-alumina fibers (10 μm in average fiberdiameter, 100 μm in average fiber length, and of an aspect ratio of 10),and the inorganic binder is silica sol (of a solid concentration of 30wt %). Further, Table 2 shows a 5 collection of data such as numericalvalues on the inorganic fibers (type, diameter, length, and aspectratio), unit shape, and unit cross-sectional area of each of Examples 30through 34 to be described below. TABLE 2 UNIT²⁾ CROSS- INORGANIC FIBERSUNIT SECTIONAL DIAMETER LENGTH ASPECT SHAPE AREA SAMPLE¹⁾ TYPE μm μmRATIO cm cm² EXAMPLE 1 SILICA-ALUMINA FIBERS 10 100 10 3.43 SQUARE 11.8EXAMPLE 30 SILICA-ALUMINA FIBERS 5 50 10 3.43 SQUARE 11.8 EXAMPLE 31SILICA-ALUMINA FIBERS 10 20 2 3.43 SQUARE 11.8 EXAMPLE 32 SILICA-ALUMINAFIBERS 10 5000 500 3.43 SQUARE 11.8 EXAMPLE 33 SILICA-ALUMINA FIBERS 1010000 1000 3.43 SQUARE 11.8 EXAMPLE 34 SILICA-ALUMINA FIBERS 10 200002000 3.43 SQUARE 11.8¹⁾CERAMIC PARTICLES = γ-ALUMINA PARTICLES²⁾UNIT AREA RATIO = 93.5% SEAL MATERIAL LAYER + COATING MATERIAL LAYERAREA RATIO = 6.5%

In each sample shown in Table 2, the ceramic particles are γ-aluminaparticles, the inorganic binder is silica sol (of a solid concentrationof 30 wt %), the unit area ratio is 93.5%, and the seal material layerarea ratio is 6.5%. Further, Table 3 shows a collection of data such asnumerical values on the inorganic binder type, unit cross-sectionalarea, seal material layer thickness, unit area ratio, seal materiallayer area ratio, and firing temperature of the honeycomb units 11 ofthe honeycomb unit 10 of each of Examples 44-51. TABLE 3 UNIT SEALSEAL²⁾ CROSS- MATERIAL UNIT MATERIAL INORGANIC SECTIONAL LAYER AREALAYER AREA FIRING BINDER AREA THICKNESS RATIO RATIO TEMPERATURE SAMPLE¹⁾TYPE cm² mm % % ° C. EXAMPLE 44 SILICA SOL 11.8 2.0 89.3 10.7 800EXAMPLE 45 SILICA SOL 11.8 3.0 84.8 15.2 800 EXAMPLE 46 SILICA SOL 5.02.0 83.5 16.5 800 EXAMPLE 47 SILICA SOL 5.0 1.5 86.8 13.2 800 EXAMPLE 48ALUMINA SOL 11.8 1.0 93.5 6.5 800 EXAMPLE 49 SEPIOLITE 11.8 1.0 93.5 6.5800 EXAMPLE 50 ATTAPULGITE 11.8 1.0 93.5 6.5 800 EXAMPLE 51 — 11.8 1.093.5 6.5 1000¹⁾CERAMIC PARTICLES = γ-ALUMINA PARTICLES INORGANIC FIBERS =SILICA-ALUMINA FIBERS (10 μm IN DIAMETER, 100 μm IN LENGTH, ASPECT RATIOOF 10)²⁾INCLUDING AREA OF COATING MATERIAL LAYER

In each sample shown in Table 3, the ceramic particles are γ-aluminaparticles (2 μm in average particle size), and the inorganic fibers aresilica-alumina fibers (10 μm in average fiber diameter, 100 μm inaverage fiber length, and of an aspect ratio of 10).

In each example of Tables 1-3, the radius of curvature R of the corners18 of the honeycomb units 11 was 1.5 mm.

EXAMPLES A THROUGH T

In Examples A through J, the honeycomb structures 10 were made in thesame manner as in Example 1 except that extrusion molding was performedwith different molds so that the rounded-surface shapes of the corners18 of the honeycomb units 11 had predetermined radii of curvature R,thereby varying the radius of curvature R from 0 to 3.0 mm. In ExamplesK through T, the honeycomb structures 10 were made in the same manner asin Example 1 except that extrusion molding was performed with differentmolds so that the corners 18 of the honeycomb units 11 had predeterminedchamfered-surface shapes, thereby changing the chamfered-surface shapesof the corners 18 of the honeycomb units 11 from 0 to 3.0 mm. InExamples J and T, the coating material layer 16 was not provided on theexternal cylindrical surface of the honeycomb structure 10. Table 4shows the radius of curvature R or the chamfered-surface shape of thecorners 18 of the honeycomb units 11 of each example in combination-withother items such as the unit cross-sectional area. In the column of unitcorner shape in Table 4, R indicates a rounded-surface shape and Cindicates a chamfered-surface shape. TABLE 4 REDUCTION RADIUS OF RATE GOF UNIT UNIT STRUCTURE CURVATURE THERMAL CROSS- UNIT SPECIFIC SPECIFICUNIT R/CHAMFERED- COATING IMPACT AND PRES- SECTIONAL AREA SURFACESURFACE CORNER SURFACE LAYER VIBRATION SURE AREA RATIO AREA AREA SHAPESHAPE THICKNESS TESTS LOSS SAMPLE cm² % m²/l m²/l R/C mm mm wt % kPaEXAMPLE A 11.8 93.5 42000 39270 R 1.5 0.5 0 2.4 EXAMPLE B 4.0 89.7 4200037674 R 1.5 0.5 0 2.8 EXAMPLE C 5.0 90.2 42000 37884 R 1.5 0.5 0 2.5EXAMPLE D 50.0 95.5 42000 40110 R 1.5 0.5 4 2.3 EXAMPLE E 55.0 95.642000 40152 R 1.5 0.5 52 2.3 EXAMPLE F 11.8 93.5 42000 39270 R 0.0 0.520 2.4 EXAMPLE G 11.8 93.5 42000 39270 R 0.3 0.5 2 2.4 EXAMPLE H 11.893.5 42000 39270 R 2.5 0.5 1 2.4 EXAMPLE I 11.8 93.5 42000 39270 R 3.00.5 29 2.5 EXAMPLE J 11.8 93.5 42000 39270 R 1.5 — 8 2.4 EXAMPLE K 11.893.5 42000 39270 C 1.5 0.5 0 2.4 EXAMPLE L 4.0 89.7 42000 37674 C 1.50.5 0 2.8 EXAMPLE M 5.0 90.2 42000 37884 C 1.5 0.5 0 2.5 EXAMPLE N 50.095.5 42000 40110 C 1.5 0.5 5 2.3 EXAMPLE O 55.0 95.6 42000 40152 C 1.50.5 55 2.3 EXAMPLE P 11.8 93.5 42000 39270 C 0.0 0.5 28 2.4 EXAMPLE Q11.8 93.5 42000 39270 C 0.3 0.5 2 2.4 EXAMPLE R 11.8 93.5 42000 39270 C2.5 0.5 0 2.4 EXAMPLE S 11.8 93.5 42000 39270 C 3.0 0.5 21 2.5 EXAMPLE T11.8 93.5 42000 39270 C 1.5 — 9 2.4

EXAMPLES 2 THROUGH 7

The honeycomb structures 10 were made in the same manner as in Example 1except that the honeycomb units 11 were made into respective shapesshown in Table 1. The shapes of the joint bodies of Examples 2, 3, and 4are shown in FIGS. 4B, 4C, and 4D, respectively, and the shapes of thejoint bodies of Examples 5, 6, and 7 are shown in FIGS. 5A, 5B, and 5C,respectively. In Example 7, the honeycomb structure 10 wasmonolithically formed, and accordingly, the joining process and thecutting process were not performed.

EXAMPLES 8 THROUGH 14

The honeycomb units 11 were made in the same manner as in Example 1except that the honeycomb units 11 were made into respective shapesshown in Table 1 using titania particles (2 μm in average particle size)instead of ceramic particles. Then, the honeycomb structures 10 weremade in the same manner as in Example 1 except that the ceramicparticles of the seal material layer 14 and the coating material layer16 were replaced with titania particles (2 μm in average particle size).The joint bodies of Examples 8 through 11 are equal in shape to those ofFIGS. 4A through 4D, respectively. The joint bodies of Examples 12through 14 are equal in shape to those of FIGS. 5A through 5C,respectively. In Example 14, the honeycomb unit 10 was monolithicallyformed.

EXAMPLES 15 THROUGH 21

The honeycomb units 11 were made in the same manner as in Example 1except that the honeycomb units 11 were made into respective shapesshown in Table 1 using silica particles (2 μm in average particle size)instead of ceramic particles. Then, the honeycomb structures 10 weremade in the same manner as in Example 1 except that the ceramicparticles of the seal material layer 14 and the coating material layer16 were replaced with silica particles (2 μm in average particle size).The joint bodies of Examples 15 through 18 are equal in shape to thoseof FIGS. 4A through 4D, respectively. The joint bodies of Examples 19through 21 are equal in shape to those of FIGS. 5A through 5C,respectively. In Example 21, the honeycomb unit 10 was monolithicallyformed.

EXAMPLES 22 THROUGH 28

The honeycomb units 11 were made in the same manner as in Example 1except that the honeycomb units 11 were made into respective shapesshown in Table 1 using zirconia particles (2 μm in average particlesize) instead of ceramic particles. Then, the honeycomb structures 10were made in the same manner as in Example 1 except that the ceramicparticles of the seal material layer 14 and the coating material layer16 were replaced with zirconia particles (2 μm in average particlesize). The joint bodies of Examples 22 through 25 are equal in shape tothose of FIGS. 4A through 4D, respectively. The joint bodies of Examples26 through 28 are equal in shape to those of FIGS. 5A through 5C,respectively. In Example 28, the honeycomb unit 10 was monolithicallyformed.

EXAMPLE 29

A commercially available cylindrical (143. 8 mm in diameter×150 mm inlength) cordierite honeycomb structure in which alumina serving as acatalyst carrier layer is formed inside through holes was employed asExample 29. The cell shape was hexagonal, the cell density was 62/cm²(400 cpsi), and the wall thickness was 0.18 mm. The shape of thehoneycomb structure viewed from its front surface is equal to that ofFIG. 5C.

EXAMPLES 30 THROUGH 34

The honeycomb units 11 were made in the same manner as in Example 1except that the honeycomb units 11 were designed using silica-aluminafibers of respective shapes shown in Table 2 as inorganic fibers. Then,the honeycomb structures 10 were made in the same manner as in Example 1except that the same silica-alumina fibers as in the honeycomb units 11were used for the seal material layer 14 and the coating material layer16. The joint bodies of Examples 30 through 34 are equal in shape tothat of FIG. 4A.

EXAMPLES 44 THROUGH 47

The honeycomb structures 10 were made in the same manner as in Example 1except that the cross-sectional areas of the honeycomb units 11 and thethickness of the seal material layer 14 joining the honeycomb units 11were changed as shown in Table 3. The joint bodies of Examples 44 and 45are equal in shape to that of FIG. 4A. The joint bodies of Examples 46and 47 are equal in shape to that of FIG. 4C.

EXAMPLE 48

The honeycomb structure 10 was made in the same manner as in Example 1except that the honeycomb units 11 were made using alumina sol (of asolid concentration of 30 wt %) as an inorganic binder as shown in Table3.

EXAMPLES 49 AND 50

The honeycomb structures 10 were made in the same manner as in Example 1except that the honeycomb units 11 were made using sepiolite in Example49 and attapulgite in Example 50 as inorganic binders. Specifically, 40wt % of γ-alumina particles (2 μm in average particle size), 10 wt % ofsilica-alumina fibers (10 μm in average fiber diameter, 100 μm inaverage fiber length, and of an aspect ratio of 10), 15 wt % of aninorganic binder, and 35 wt % of water were mixed, and an organicbinder, a plasticizer, and lubricant were added to the obtained mixtureas in Example 1. Then, molding and firing were performed, so that thehoneycomb units 11 were obtained. Next, the honeycomb units 11 werejoined with the same seal material paste as that of Example 1. Then, asin Example 1, the obtained joint body was cut and the coating materiallayer 16 was formed thereon, so that the cylindrical (143. 8 mm indiameter×150 mm in length) honeycomb structure 10 was obtained.

EXAMPLE 51

The honeycomb structure 10 was made in the same manner as in Example 1except that the honeycomb units 11 were made without putting in aninorganic binder. Specifically, 50 wt % of γ-alumina particles (2 μm inaverage particle size), 15 wt % of silica-alumina fibers (10 μm inaverage fiber diameter, 100 μm in average fiber length, and of an aspectratio of 10), and 35 wt % of water were mixed, and as in Example 1, anorganic binder, a plasticizer, and lubricant were added to the obtainedmixture, and molding was performed. The obtained molded bodies weresubjected to firing at 1000° C., so that the honeycomb units 11 wereobtained. Next, the honeycomb units 11 were joined with the same sealmaterial paste as that of Example 1. Then, as in Example 1, the obtainedjoint body was cut and the coating material layer 16 was formed thereon,so that the cylindrical (143. 8 mm in diameter×150 mm in length)honeycomb structure 10 was obtained.

[Specific Surface Area Measurement]

The specific surface areas of the honeycomb units 11 of Examples 1through 51 and Examples A through T were measured. First, the volumes ofthe honeycomb units 11 and the seal material were actually measured, andthe ratio of the unit material to the volume of the honeycomb structure10 (=A [vol %]) was calculated. Next, the BET specific surface area perunit weight of the honeycomb units 11 (=B [m²/g]) was measured. The BETspecific surface area was measured by the single-point method based onJIS-R-1626 (1996) set by Japanese Industrial Standards using a BETmeasuring apparatus (Micromeritics FlowSorb II-2300 manufactured byShimadzu Corporation). In the measurement, a cylindrically cut-out smallpiece (15 mm in diameter×15 mm in height) was employed as a sample. Theapparent density of the honeycomb units 11 (=C [g/L]) was calculatedfrom the weight and the volume of the outer shape of the honeycomb units11, and the specific surface area of the honeycomb structure 10 (=S[m²/L]) was given by the following Eq. (1). Here, the specific surfacearea of the honeycomb structure 10 refers to the specific surface areaper apparent volume of the honeycomb structure 10.S(m ² /L)=(A/100)×B×C.   (1)

The contents of JIS-R-1626 (1996) are incorporated herein by referencein their entirety.

[Repeated Thermal Shock and Vibration Tests]

The honeycomb structures 10 of Examples 1 through 51 and Examples Athrough T were subjected to repeated thermal shock and vibration tests.In the thermal shock test, each honeycomb structure 10 in a metal casing21 (FIGS. 6A and 6B) with an alumina mat (MAFTEC, manufactured byMitsubishi Chemical Functional Products, Inc., 46.5 cm×15 cm, 6 mm inthickness), which is a heat insulator formed of alumina fibers, beingaround the exterior cylindrical surface of the honeycomb structure 10,was put in a furnace set at 600° C. After being heated for 10 minutes,the honeycomb structure 10 in the metal casing 21 was extracted from thefurnace and cooled rapidly to room temperature. Next, the vibration testwas conducted on the honeycomb structure 10 in the metal casing 21. FIG.6A is a front view of a vibration apparatus 20 used for the vibrationtest. FIG. 6B is a side view of the vibration apparatus 20. The metalcasing 21 containing the honeycomb structure 10 was placed on a seat 22,and the metal casing 21 was fixed by fastening substantially U-shapedfixtures 23 with screws 24. As a result, the metal casing 21 was capableof vibrating with the seat 22 and the fixtures 23 as a unit. Withrespect to Examples 1 through 51, the vibration test was conducted underthe conditions of a frequency of 160 Hz, an acceleration of 30 G, anamplitude of 0.58 mm, a retention time of 10 hours, room temperature,and the vibrating directions along the Z-axis (upward and downward). Onthe other hand, with respect to Examples A through T, the retention timewas 20 hours. These thermal shock test and vibration test were repeatedalternately ten times each, and the weight of the honeycomb structure 10before the tests (=TO) and the weight of the honeycomb structure afterthe tests (=Ti) were measured, so that the rate of weight reduction (=G)was determined using the following Eq. (2).G(wt %)=100×(T0−Ti)/T0.   (2)

[Pressure Loss Measurement]

The pressure loss of each of the honeycomb structures 10 of Examples 1through 51 and Examples A through T was measured. FIG. 7 is a schematicdiagram showing a pressure loss measuring apparatus 40. The measurementwas performed as follows. The honeycomb structure 10 having an aluminamat wrapped therearound in a metal casing was placed in the exhaust pipeof a 2L common rail diesel engine 42, and a differential pressure gauge44 was attached across the honeycomb structure 10. The measurementconditions were an engine speed of 1500 rpm and a torque of 50 Nm, andthe differential pressure at 5 minutes after the start of operation wasmeasured.

[Experimental Results]

Table 5 shows a collection of data such as numerical values on theceramic particle component(s), unit cross-sectional area, unit arearatio, the specific surface area of the honeycomb unit 11, the specificsurface area S of the honeycomb structure 10, the weight reduction rateG of the repeated thermal shock and vibration tests, and pressure lossof each of Examples 1 through 29 and 44 through 47. FIG. 8 is a graphshowing the cross-sectional area of the honeycomb unit 11 plotted as thehorizontal axis, and the weight reduction rate G of the repeated thermalshock and vibration tests and the pressure loss plotted as the verticalaxes. FIG. 9 is a graph showing the unit area ratio plotted as thehorizontal axis, and the weight reduction rate G of the repeated thermalshock and vibration tests and the pressure loss plotted as the verticalaxes. TABLE 5 REDUCTION RATE G OF UNIT UNIT STRUCTURE THERMAL CROSS-UNIT SPECIFIC SPECIFIC SHOCK AND SECTIONAL AREA SURFACE SURFACEVIBRATION PRESSURE CERAMIC AREA RATIO AREA AREA S TESTS LOSS SAMPLE^(※)PARTICLES cm² % m²/L m²/L wt % kPa EXAMPLE 1 ALUMINA 11.8 93.5 4200039270 0 2.4 EXAMPLE 2 ALUMINA 4.0 89.7 42000 37674 0 2.8 EXAMPLE 3ALUMINA 5.0 90.2 42000 37884 0 2.5 EXAMPLE 4 ALUMINA 39.5 96.9 4200040698 5 2.2 EXAMPLE 5 ALUMINA 50.0 95.5 42000 40110 3 2.3 EXAMPLE 6ALUMINA 55.0 95.6 42000 40152 52 2.3 EXAMPLE 7 ALUMINA 162.0 100.0 4200042000 70 2.1 EXAMPLE 8 TITANIA 11.8 93.5 38000 35530 0 2.4 EXAMPLE 9TITANIA 4.0 89.7 38000 34086 0 2.8 EXAMPLE 10 TITANIA 5.0 90.2 3800034276 0 2.5 EXAMPLE 11 TITANIA 39.5 96.9 38000 36822 7 2.2 EXAMPLE 12TITANIA 50.0 95.5 38000 36290 5 2.3 EXAMPLE 13 TITANIA 55.0 95.6 3800036328 63 2.3 EXAMPLE 14 TITANIA 162.0 100.0 38000 38000 90 2.1 EXAMPLE15 SILICA 11.8 93.5 41000 38335 0 2.4 EXAMPLE 16 SILICA 4.0 89.7 4100036777 0 2.8 EXAMPLE 17 SILICA 5.0 90.2 41000 36982 0 2.5 EXAMPLE 18SILICA 39.5 96.9 41000 39729 4 2.2 EXAMPLE 19 SILICA 50.0 95.5 4100039155 3 2.3 EXAMPLE 20 SILICA 55.0 95.6 41000 39196 42 2.3 EXAMPLE 21SILICA 162.0 100.0 41000 41000 65 2.1 EXAMPLE 22 ZIRCONIA 11.8 93.541500 38803 0 2.4 EXAMPLE 23 ZIRCONIA 4.0 89.7 41500 37226 0 2.8 EXAMPLE24 ZIRCONIA 5.0 90.2 41500 37433 0 2.5 EXAMPLE 25 ZIRCONIA 39.5 96.941500 40214 5 2.2 EXAMPLE 26 ZIRCONIA 50.0 95.5 41500 39633 3 2.3EXAMPLE 27 ZIRCONIA 55.0 95.6 41500 39674 57 2.3 EXAMPLE 28 ZIRCONIA162.0 100.0 41500 41500 83 2.1 EXAMPLE 29 CORDIERITE + 162.0 100.0 2500025000 0 2.9 ALUMINA EXAMPLE 44 ALUMINA 11.8 89.3 42000 37506 0 3.1EXAMPLE 45 ALUMINA 11.8 84.8 42000 35616 0 4.3 EXAMPLE 46 ALUMINA 5.083.5 42000 35070 0 4.4 EXAMPLE 47 ALUMINA 5.0 86.8 42000 36456 0 3.3^(※)INORGANIC FIBERS = SILICA-ALUMINA FIBERS (10 μm IN DIAMETER, 100 μmIN LENGTH, ASPECT RATIO OF 10)

The measurement results of Examples 1 through 29 and 44 through 47 shownin Table 5 and FIG. 8 clearly show that with ceramic particles,inorganic fibers, and an inorganic binder being employed as principalcomponents and the cross-sectional area of the honeycomb unit 11 beingwithin 5-50 cm², the honeycomb structure 10 has a large specific surfacearea per unit volume and sufficient strength against thermal shock andvibration. Further, it has also been found that as shown in FIG. 9, withceramic particles, inorganic fibers, and an inorganic binder beingemployed as principal components, the cross-sectional area of thehoneycomb unit 11 being less than or equal to 50 cm², and the unit arearatio being greater than or equal to 85%, the honeycomb structure 10 hasa large specific surface area per unit volume, sufficient strengthagainst thermal shock and vibration, and reduced pressure loss. Inparticular, when the unit area ratio is greater than or equal to 90%,the pressure loss decreases conspicuously.

Table 4 shows a collection of the results of the weight reduction rate Gof the repeated thermal shock and vibration tests and the pressure lossof Examples A through J, in which the radius of curvature R of thecorners 18 of the honeycomb units 11 was varied, and Examples K throughT, in which the chamfered-surface shapes of the corners 18 of thehoneycomb units 11 were changed. These results show that the honeycombstructure 10 has excellent strength when the radius of curvature R ofthe corners 18 of the honeycomb units 11 falls within the range of0.3-2.5 mm. Further, these results show that the honeycomb structure 10has excellent strength when the chamfered-surface shapes are 0.3-2.5 mm.It is easily inferred that the same effect can also be produced when thecorners 18 of the single honeycomb unit 11 include both rounded-surfaceand chamfered-surface shapes within the above-described ranges.

Next, Table 6 shows a collection of numerical data on the diameter,length, and aspect ratio of the silica-alumina fibers; the specificsurface area of the honeycomb unit 11; the specific surface area S ofthe honeycomb structure 10; the weight reduction rate G of the repeatedthermal shock and vibration tests; and the pressure loss of Examples 1and 30 through 34 in which the aspect ratio of inorganic fibers wasvaried. FIG. 10 is a graph showing the aspect ratio of thesilica-alumina fibers plotted as the horizontal axis, and the weightreduction rate G of the repeated thermal shock and vibration testsplotted as the vertical axis with respect to Examples 1 and 30 through34. TABLE 6 REDUCTION RATE G OF UNIT STRUCTURE THERMAL SPECIFIC SPECIFICSHOCK AND SILICA-ALUMINA FIBERS SURFACE SURFACE VIBRATION PRESSUREDIAMETER LENGTH ASPECT AREA AREA S TESTS LOSS SAMPLE^(※) μm μm RATIOm²/L m²/L wt % kPa EXAMPLE 1 10 100 10 42000 39270 0 2.4 EXAMPLE 30 5 5010 42000 39270 2 2.4 EXAMPLE 31 10 20 2 42000 39270 8 2.4 EXAMPLE 32 105000 500 42000 39270 4 2.4 EXAMPLE 33 10 10000 1000 42000 39270 6 2.4EXAMPLE 34 10 20000 2000 42000 39270 25 2.4^(※)CERAMIC PARTICLES = γ-ALUMINA PARTICLES

These results show that sufficient strength against thermal shock andvibration can be obtained when the aspect ratio of inorganic fibersfalls within the range of 2-1000.

Next, Table 7 shows a collection of data such as numerical values on theinorganic binder type, the firing temperature of the honeycomb unit 11,unit area ratio, the specific surface area of the honeycomb unit 11, thespecific surface area S of the honeycomb structure 10, the weightreduction rate G of the repeated thermal shock and vibration tests, andpressure loss of each of Examples 48 through 50, in which the honeycombunits 11 were made with different types of inorganic binders, andExample 51, in which the honeycomb units 11 were made without putting inan inorganic binder. TABLE 7 REDUCTION RATE G OF UNIT STRUCTURE THERMALUNIT SPECIFIC SPECIFIC SHOCK AND INORGANIC AREA FIRING SURFACE SURFACEVIBRATION PRESSURE BINDER RATIO TEMPERATURE AREA AREA S TESTS LOSSSAMPLE^(※) TYPE % ° C. m²/L m²/L wt % kPa EXAMPLE 48 ALUMINA SOL 93.5800 42000 39270 0 2.4 EXAMPLE 49 SEPIOLITE 93.5 800 42000 39270 0 2.4EXAMPLE 50 ATTAPULGITE 93.5 800 42000 39270 0 2.4 EXAMPLE 51 — 93.5 100042000 37400 20 2.4^(※)CERAMIC PARTICLES = γ-ALUMINA PARTICLES INORGANIC FIBERS =SILICA-ALUMINA FIBERS (10 μm IN DIAMETER, 100 μm IN LENGTH, ASPECT RATIOOF 10) UNIT SHAPE = 3.43 cm square

These results show that when no inorganic binder is put in, sufficientstrength can be obtained by firing at relatively high temperatures.Further, these results show that sufficient strength can also beobtained by firing at relatively low temperatures when an inorganicbinder is put in. These results also show that even when the inorganicbinder is alumina sol or a clay-based binder, the honeycomb structure 10can have a great specific surface area per unit volume and sufficientstrength against thermal shock and vibration.

[Honeycomb Catalyst]

Each of the honeycomb structures 10 of Examples 1 through 43 wasimpregnated with a platinum nitrate solution, and was caused to carry acatalyst component with the weight of platinum per unit volume of thehoneycomb structure 10 being controlled to 2g/L. Then, the honeycombstructure 10 was heated at 600° C. for 1 hour, so that a honeycombcatalyst was obtained.

According to one embodiment of the present invention, in a honeycombstructure having multiple honeycomb units united through a seal materiallayer, the honeycomb units each having multiple through holes arrangedside by side in a longitudinal direction and separated from each otherby the wall surfaces of the through holes, the honeycomb units includeat least ceramic particles and inorganic fibers and/or whiskers, atleast one of the honeycomb units has a cross section perpendicular to alongitudinal direction thereof, the cross section having an area greaterthan or equal to about 5 cm² and less than or equal to about 50 cm², andeach corner of each honeycomb unit has a shape of one of a substantiallyrounded surface and a substantially chamfered surface.

Thus, a honeycomb structure capable of achieving high dispersion of acatalyst component and increasing strength against thermal shock andvibration can be provided.

Additionally, in the honeycomb structure, the rounded surface of eachcorner of each honeycomb unit may have a radius of curvature of about0.3 mm to about 2.5 mm. This makes it possible to ease stressconcentration on the edge parts of the honeycomb structure, thusincreasing its strength.

Additionally, in the honeycomb structure, the shape of the chamferedsurface of each corner of each honeycomb unit may be about 0.3 mm toabout 2.5 mm in size. This makes it possible to ease stressconcentration on the edge parts of the honeycomb structure, thusincreasing its strength.

Additionally, in the honeycomb structure, the ratio of the total of theareas of the cross sections of the honeycomb units perpendicular to thelongitudinal direction thereof to the area of a cross section of thehoneycomb structure perpendicular to a longitudinal direction thereof isgreater than or equal to about 85%. This makes it possible to cause arelative increase in the surface area capable of carrying a catalyst,and a relative reduction in pressure loss.

Additionally, the honeycomb structure may include a coating materiallayer on the exterior surface thereof. This makes it possible toincrease the strength of the honeycomb structure by protecting itsexterior surface.

Additionally, in the honeycomb structure, the ceramic particles mayinclude at least one selected from alumina, silica, zirconia, titania,ceria, mullite, and zeolite. This makes it possible to increase thespecific surface area of the honeycomb units.

Additionally, in the honeycomb structure the inorganic fibers and/or thewhiskers may include at least one selected from alumina, silica, siliconcarbide, silica alumina, glass, potassium titanate, and aluminum borate.This makes it possible to increase the strength of the honeycomb units.

Additionally, in the honeycomb structure, the honeycomb units may bemanufactured using a mixture including the ceramic particles, theinorganic fibers and/or the whiskers, and an inorganic binder; and theinorganic binder may include at least one selected from alumina sol,silica sol, titania sol, water glass, sepiolite, and attapulgite. Thismakes it possible to obtain sufficient strength even if the honeycombunits are subjected to firing at low temperatures.

Additionally, a catalyst component may be carried on the honeycombstructure. As a result, a honeycomb catalyst having a catalyst componenthighly dispersed can be obtained.

Additionally, in the honeycomb structure, the catalyst component mayinclude at least one component selected from noble metal, alkali metal,alkaline earth metal, and oxide. This makes it possible to increaseconversion performance.

Additionally, the honeycomb structure may be used for converting theexhaust gas of a vehicle.

The present invention may be applied to a catalyst carrier forconverting (purifying) the exhaust gas of vehicles and an adsorbentadsorbing a gas component or a liquid component.

The present invention is not limited to the specifically disclosedembodiment, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on PCT International Application No.PCT/JP2005/011657, filed on Jun. 24, 2005, the entire contents of whichare hereby incorporated by reference.

1. A honeycomb structure having a plurality of honeycomb units unitedthrough a seal material layer, the honeycomb units each having aplurality of through holes arranged side-by side in a longitudinaldirection and separated from each other by wall surfaces of the throughholes, wherein: the honeycomb units comprise at least: ceramicparticles; and at least one of inorganic fibers and whiskers; at leastone of the honeycomb units has a cross section perpendicular to alongitudinal direction thereof, the cross section having an area greaterthan or equal to about 5 cm² and less than or equal to about 50 cm^(2;)and each of corners of each honeycomb unit has a shape of one of asubstantially rounded surface and a substantially chamfered surface. 2.The honeycomb structure as claimed in claim 1, wherein the roundedsurface of each corner of each honeycomb unit has a radius of curvatureof about 0.3 mm to about 2.5 mm.
 3. The honeycomb structure as claimedin claim 1, wherein the shape of the chamfered surface of each corner ofeach honeycomb unit is about 0.3 mm to about 2.5 mm in size.
 4. Thehoneycomb structure as claimed in claim 1, wherein a ratio of a total ofthe areas of the cross sections of the honeycomb units perpendicular tothe longitudinal direction thereof to an area of a cross section of thehoneycomb structure perpendicular to a longitudinal direction thereof isgreater than or equal to about 85%.
 5. The honeycomb structure asclaimed in claim 1, further comprising: a coating material layer on anexterior surface of the honeycomb structure.
 6. The honeycomb structureas claimed in claim 1, wherein the ceramic particles comprise at leastone selected from the group consisting of alumina, silica, zirconia,titania, ceria, mullite, and zeolite.
 7. The honeycomb structure asclaimed in claim 1, wherein the at least one of the inorganic fibers andthe whiskers comprise at least one selected from the group consisting ofalumina, silica, silicon carbide, silica alumina, glass, potassiumtitanate, and aluminum borate.
 8. The honeycomb structure as claimed inclaim 1, wherein: the honeycomb units are manufactured using a mixtureincluding the ceramic particles, the at least one of the inorganicfibers and the whiskers, and an inorganic binder; and the inorganicbinder comprises at least one selected from the group consisting ofalumina sol, silica sol, titania sol, water glass, sepiolite, andattapulgite.
 9. The honeycomb structure as claimed in claim 1, wherein acatalyst component is carried on the honeycomb structure.
 10. Thehoneycomb structure as claimed in claim 9, wherein the catalystcomponent comprises at least one component selected from the group ofnoble metal, alkali metal, alkaline earth metal, and oxide.
 11. Thehoneycomb structure as claimed in claim 1, wherein the honeycombstructure is used for conversion of exhaust gas of a vehicle.
 12. Thehoneycomb structure as claimed in claim 1, wherein a number of thethrough holes per unit cross-sectional area is about 15.5/cm² to about186/cm².
 13. The honeycomb structure as claimed in claim 1, wherein theseal material layer is about 0.5 mm to about 2 mm in thickness.
 14. Thehoneycomb structure as claimed in claim 1, wherein a wall between thethrough holes is about 0.05 mm to about 0.35 mm in thickness.